API comment conventions

[Rendered](https://github.com/steveklabnik/rfcs/blob/api_comment_conventions/text/0000-api-comment-conventions.md)

I'm mostly trying to codify what we already have consensus on, rather than address every possible question. I can forsee this kind of RFC getting very bogged down in a number of different details, and we can always add more things in a future RFC.
@steveklabnik sure, there's various ways of doing a 'proper' citation (I don't even know what they are/how to write them by hand; I just use whatever BibTeX spits out), but that doesn't mean we should just give up on a long form entirely. :)

I guess we could chose one of the formal specifications, but it seems extremely low-priority since we don't cite things particularly often.